This invention relates to a method for passivation of cleaned steel surfaces, and particularly to an effective method for preventing of flash rusting of cleaned wet steel that is drying prior to the application of a non-aqueous protective coating.
It is well known that steel surfaces need to be dry before the application of solvent-based coatings, such as epoxy and urethane high performance coatings for example, because such coatings will not adhere well to moist surfaces and water on the surfaces will not readily dissolve in or be displaced by the applied solvent-based formulated coatings. The drying of cleaned steel after rinsing with water readily produces flash rusting, especially when the humidity is high or the temperature is elevated.
It is also well known that water-soluble residues, and especially ionic water-soluble residues, remaining on a steel surface after application of a protective coating, will decrease coating performance and increase corrosion. Therefore, the cleaned steel surfaces should be free of such residues when the protective coating is applied.
Furthermore, it is well known that rusting is inhibited in an alkaline environment. Thus, water-soluble amines are sometimes added to latex or water-dispersed coatings for steel to reduce corrosion. Water-soluble amines also have been added to final rinses for cleaned steel, but always in combination with other materials (such as other alkaline chemicals, citric acid, sodium nitrite, etc., and as exemplified in U.S. Pat. Nos. 3,072,502; 3,154,438; 3,368,913; 3,519,458; and 4,045,253) and therefore these rinses have left insoluble residues on the steel surfaces that are detrimental to optimum performance of subsequently applied protective coatings.
In the prior art, cleaned steel is often passivated by treating with an alkaline sodium nitrite solution to provide altered surface characteristics that resist rusting; for unknown reasons, this method sometimes does not work. Dilute solutions of citric acid made alkaline with ammonia or with an amine have been used for passivation of cleaned steel surfaces. These same solutions also have been used in combination with sodium nitrite. A disadvantage of all these methods is that when the residual passivation solution evaporates, water-soluble materials remain behind which can affect the adhesion and the water resistance of subsequently applied non-aqueous protective coatings. Mixing sodium nitrite passivation solution with an acid solution, such as that remaining on steel surfaces following a cleaning process, will result in formation of hazardous, volatile toxic oxides of nitrogen; this also is avoided by the present invention.